济源盆地中侏罗统马凹组砂岩粒度特征分析

Grain size analysis for the sandstones from the Middle Jurassic Ma'ao Formation in the Jiyuan Basin,Henan

济源盆地中侏罗统马凹组下段发育厚约80m的中细粒砂岩,前人在许多文献中称其为长石石英砂岩。通过对所取砂岩样品薄片镜下观察,将其命名为岩屑石英砂岩。该段砂岩自下而上粒度具有以下特征:平均粒径(Mz)总体由大变小,标准偏差(σ)反映砂岩分选由中等逐渐变好,标准偏差(σ)和偏度(SK1)都具有典型河流沉积物特征。频率曲线表现为不对称三峰或双峰,概率累积曲线由"低斜四段"式渐变为"高斜跳悬两段"式。该套砂岩沉积时期水流不稳,有多期洪流出现,表现为平均粒径和标准偏差的局部来回振荡。其C-M图显示主要发育牵引流PQ、QR、RS段,符合经典曲流河沉积模式。

About 80-m thick medium- to fine-grained sandstones are recognized in the lower member of the Middle Jurassic Ma＇ao Formation in the Jiyuan Basin, Henan, and previously were considered as the feldspathic quartz sandstones in many literatures. The authors in the present paper redetermined them as lithic quartz sandstones. These sandstones become progressively smaller in mean grain size （Mz） and better in sorting. Moreover, the standard deviation （σ） and skewness （SKI） for the sandstones suggest the typical features of the fluvial deposits. The frequency curves display asymmetrical bimodal or trimodal patterns, and the probability cumulative curves display the gradations from ＂low oblique four-part＂ patterns to the ＂ high oblique two-part＂ patterns. Multiphase floods during the deposition of the sandstones are responsible for the partial oscillation of the mean grain sizes and standard deviations. In addition, the occurrence of the PQ, QR and RS intervals of the tractional currents indicated in the CM patterns also shows the classical patterns of the meandering stream deposits.